In internal combustion engines with externally supplied ignition there may be an increased tendency to pre-ignition, in particular in conjunction with high-pressure supercharging. An irregular combustion process in which portions of the fuel/air mixture are ignited in the combustion chamber even before the initiation of ignition by means of a spark plug is referred to as pre-ignition. Pre-ignitions as a rule are associated with a combustion-chamber pressure which is so greatly increased that damage may occur to the internal combustion engine. Knocking combustions are likewise already known as irregular combustion processes, but these are characterised in that after the initiation of an ignition by means of a spark plug portions of the not yet combusted fuel/air mixture are self-ignited, with the augmented flame fronts which are now present striking one another and causing the high-frequency cylinder-pressure vibrations characteristic of knocking combustion. Pre-ignitions may occur as early as in the compression phase. The causes of pre-ignitions are complex. Known causes which lead, individually or in combination, to pre-ignitions are self-ignitions at hot exposed points in the combustion chamber, charge inhomogeneities and an increased temperature level in the combustion chamber. Inter alia, the parameters of the charge cycle, such as valve timings, the amount of fuel or the combustion-air ratio, the charging pressure, the ignition-angle setting or parameters of the exhaust recirculation, have an influence on pre-ignitions. Externally-induced pre-ignitions, i.e. combustions which are triggered neither purely by reaction kinetics nor by the active ignition, are also referred to as auto-ignitions. For safe operation of an internal combustion engine, it is necessary to recognise every pre-ignition, both ones triggered by reaction kinetics and ones triggered by external induction, in order to take countermeasures immediately and to avoid further pre-ignitions.
A method for recognising pre-ignitions which is based on combined evaluation of the rate-of-rotation signal of the crankshaft and the signal of a knock sensor is already known from DE 10 2007 024 415 B3. If the rate of rotation of the crankshaft in the compression phase is reduced compared with a comparison value and the signal of the knock sensor indicates knocking combustion, it is concluded that pre-ignition is occurring. This method departs from the assumption that a pre-ignition/auto-ignition generates similar high-frequency pressure vibrations in the combustion chamber to knocking combustion. Tests have shown that this assumption is not always correct, i.e. no high-frequency vibrations are accompanied by pre-ignition. Furthermore, the effect of a pre-ignition on the rate-of-rotation signal of the crankshaft is slight and, in particular at a high rate of rotation of the crankshaft, detection of pre-ignitions is extremely difficult or even impossible, since the interference on the rate-of-rotation signal increases greatly.